Why am I getting a null pointer when adding to the end of a linked list

Question

I am having problems with my add() method in my linked list implementation. It was working fine until I added the method, doubleInPlace(). It seems that the problem has something to do with how I have myTail implemented. Could someone provide any advice on how to solve this issue?

public void add (Object x) {
    ListNode temp;
    if(isEmpty()){
        myHead = new ListNode(x);
        myTail = myHead;
    }
    else{
    ListNode addNode = new ListNode(x);
    myTail.setNext(addNode);
    myTail = myTail.myNext;
    }
    mySize++;
}

Here is the full implementation of my linked list.

import java.util.*; public class List implements Iterable{

private ListNode myHead;
private ListNode myTail;
private int mySize;
public List() {
    myHead = null;
    myTail = null;
    mySize = 0;
}

public boolean isEmpty() {
    return myHead == null;
}

private static class ListNode {

    private Object myItem;
    private ListNode myNext;

    private ListNode (Object item, ListNode next) {
        myItem = item;
        myNext = next;
    }

    private ListNode (Object item) {
        myItem = item;
        myNext = null;
    }

    public void setNext(Object item){
        myNext = new ListNode(item);
    }

    public void setNext(ListNode node){
        myNext = node;
    }

    public ListNode getNext(){
        return myNext;
    }
    public void setItem(Object item){
        myItem = item;
    }

}

public Iterator iterator() {
    return new ElementIterator();
}

public class ElementIterator implements Iterator {
    ListNode p;
    int nextElement;
    int size;
    public ElementIterator() {
        p = myHead;
        nextElement = 0;
    }

    public boolean hasNext() {
        if (isEmpty()){
            return false;
        }
        return nextElement < size();

    }

    public Object next() {
        Object next = p.myItem;
        if (nextElement < size()){
            next = get(nextElement);
            nextElement++;
        }
        return next;
    }

    public void remove() {
        // not used
    }
}

public String toString() {
    String rtn = "( ";
    for (ListNode p = myHead; p != null; p = p.myNext) {
        rtn = rtn + p.myItem + " ";
    }
    return rtn + ")";
}

// Return the number of items in this list ("length" in Scheme).
public int size() {
    return mySize;
}

// Return true if the list contains the object 
public boolean contains (Object obj) {
    for (ListNode p = myHead; p != null; p = p.myNext) {
        if (obj.equals (p.myItem)) {
            return true;
        }
    }
    return false;
}

// Returns the element at the given position in this list.
public Object get (int pos) {
    if (pos < 0) {
        throw new IllegalArgumentException (
                "Argument to get must be at least 0.");
    }
    if (pos >= size()) {
        throw new IllegalArgumentException ("Argument to get is too large.");
    }
    int k = 0;
    for (ListNode p = myHead; p != null; p = p.myNext) {
        if (k == pos) {
            return p.myItem;
        }
        k++;
    }
    return null;
}

public void addToFront (Object obj) {
    myHead = new ListNode (obj, myHead);
    mySize++;
}

public boolean equals (Object obj) {
    List comp = (List) obj;
    ListNode comp_node;
    Object compare_node;
    boolean isEqual = false;
    int count = 0;
    if(size()!=comp.size()){
        return false;
    }
    if(obj.toString().equals(this.toString())){
        return true;
    }
    return isEqual;
}

/*public void add (Object x) {
    ListNode temp;
    if(isEmpty()){
        myHead = new ListNode(x);
    }
    else{
    temp = new ListNode(x);
    ListNode orig = this.myHead;
    for (; orig.myNext != null; orig = orig.myNext){
        //orig = orig.myNext;
    }
    //while (orig.myNext != null){
    //  orig = orig.myNext;
    //}
    orig.myNext = new ListNode(x);
    }
    mySize++;
} */

public void add (Object x) {
    ListNode temp;
    if(isEmpty()){
        myHead = new ListNode(x);
        myTail = myHead;
    }
    else{
    ListNode addNode = new ListNode(x);
    //myTail.myNext = new ListNode(x, null);
    myTail.setNext(addNode);
    myTail = myTail.myNext;
    }
    mySize++;
}


public void appendInPlace (List l) {
    if(isEmpty()){
        myHead = l.myHead;
        myTail = l.myTail;
    }else if(l.isEmpty()){
        return;
    }
    else{
        myTail.myNext = l.myHead;
        myTail = l.myTail;
    }
}

//remove all links that are equal to object a
public void remove(Object a){
    if (isEmpty()){
        return;
    }
    if (myHead.myItem == a){
        myHead = myHead.myNext;
        mySize--;
    }

    //loop through the nodes and if item equals a, set item to next item and ref to next's next
    ListNode p = myHead;
    ListNode lastNode;
    lastNode = new ListNode("");
    int tempSize = mySize;
    for (int i=0; i < tempSize - 1; i++){
        if (p.myItem == a){
            lastNode.setNext(p.myNext);
            mySize--;
        }
        lastNode = p;
        p = p.myNext;
    }

    //if tail is equal to item, delete last node
    removeTail(a);      
}

public void removeTail(Object a){
    if (myTail.myItem == a){
        ListNode temp = myHead;
        ListNode temp2 = myTail;

        if (mySize == 2){
            temp.myNext = null;
            myTail = temp;
        }

        for (int i = 0; i< mySize-2; i++){
            temp = temp.myNext;
        }

        temp.myNext = null;
        myTail = temp;
        mySize--;

    }       
}
public void doubleInPlace() {
    ListNode p = myHead;
    for (; p != null; p = p.myNext) {
        ListNode temp = new ListNode(p.myItem, p.myNext);
        p.setNext(temp);
        p= p.myNext;
    }
    myTail = p;
}

public void isOK(){
    boolean isOK = true;
    //Check size is ok
    int mySizeCheck = 0;
    ListNode temp = myHead;
    while (temp != null){
        temp = temp.myNext;
        mySizeCheck++;
    }
    if (temp.myItem != myTail.myItem){
        throw new IllegalStateException("The tail is not correct.");
    }
    if (mySize != mySizeCheck){
        throw new IllegalStateException("The size is not correct.");
    }

}

}